1. Field of Invention
The present invention relates to a global positioning system (GPS), and more particularly to a method for calculating current position coordinate applicable to a GPS receiver.
2. Related Art
Global navigation satellite system is also called GPS. In the past, the GPS is only limited to military purpose and industrial purpose. With the continuous development of the technology, the GPS begins to be applied to various civil purposes. Generally, the GPS products mainly refer to GPS receivers applied to various purposes, for example, receivers for aviation and voyage purposes, automobile navigation devices, hand-held receivers for mountaineering and entertainment, and other types of communication products. The common GPS product mainly includes an internal antenna, a chipset, external keys, a display panel, and other related parts.
The GPS receiver determines positions by utilizing satellites surrounding the earth.
When each satellite operates, one coordinate value representing the position (known) exists at any moment. The position coordinate of each satellite is provided by a respective ephemeris data of each satellite.
The position coordinate of the GPS receiver is unknown. The GPS receiver compares a clock of a satellite signal of each satellite and a clock generated in the GPS receiver, so as to calculate the time cost during a process of transmitting the satellite signal. The time difference (that is, the time cost during the process of transmitting the satellite signal) value is multiplied by an electric wave transmission speed (generally the speed of light), so as to calculate a distance between the satellite and the GPS receiver, which is called a pseudo range. According to the triangle vector, a related equation is given for the pseudo range.
When the satellite signal of one satellite is received, one related equation is given. Therefore, when the satellite signals of three satellites are received, a plane coordinate (that is, longitude value and latitude value) is calculated. The calculated plane coordinate is the position coordinate of the GPS receiver. When the satellite signals of four satellites are received, in addition to the plane coordinate (that is, the longitude value and the latitude value), a height value is also calculated. Further, if the satellite signals of more than five satellites are utilized, the accuracy of the calculated position coordinate is improved.
The satellite repeatedly broadcasts the satellite signal once at each millisecond (ms), such that the GPS receiver calculates a phase difference of the satellite by utilizing a phase lock loop (PLL) at each ms, and calculates a translation amount of the satellite by utilizing a delay lock loop (DLL). Then, the GPS receiver corrects a carrier frequency by utilizing the calculated phase difference, and corrects a code delay by utilizing the calculated translation amount. When positioning, the GPS receiver calculates the position coordinate thereof by utilizing the code delay at that time.
Generally, the GPS receiver re-calculates the position coordinate at each second, such that the position coordinate of the GPS receiver at each second are the latest.
In other words, in order to continuously track the position of the satellite, the GPS receiver needs to perform a great amount of operation (1000 times) at each second, so as to ensure that the position coordinate calculated at each second is correct.